Hi, Haribon, Welcome to the forum. You'll get a lot of good advice here, we are a friendly bunch (usually)!
Most caminos are very crowded in Holy Week, because many Spaniards are able to get the week off. But Holy Week is a great time to walk, because of all the religious pageantry, so that's the dilemma. If you are looking to avoid crowds, I think that starting on Easter Sunday is the better choice, but then you will miss the Holy Week processions. So your plan about splitting the difference may be the best choice.
And let me make a comment about pack transfer. If I were you, I would be really careful about carrying a few things in a day pack while someone else transports your backpack. I have lower back problems, too, but I carry my internal frame backpack with 15 kilos with not a problem in the world. But give me a day pack and a few kilos of water, food, and a raincoat and my back is very unhappy within an hour or two. So be careful. I have found that walking with a good backpack, with internal frame support, actually helps my back.